1. Field of the Invention
The present invention relates generally to a color video signal reproducing apparatus, and more particularly to a color video signal reproducing apparatus in which an automatic chroma (color) control (which will be hereinafter referred simply to as an ACC) operation is positively performed and also in which erroneous operation of a color killer caused by the ACC operation is avoided.
2. Description of the Prior Art
In the recording operation of a prior art VTR (color video signal magnetic recording and/or reproducing apparatus or video tape recorder), a luminance signal is converted into an FM signal which may occupy the higher portion of a band which is recordable, a chrominance signal is frequency-converted to occupy a portion of the recordable band at the lower side of the band of the FM luminance signal, and the frequency-converted chrominance signal and the FM luminance signal are combined to form a combined signal which is recorded.
The reproducing system of such prior art VTR is formed as shown in FIG. 1, in which reference numerals or symbols 1A and 1B indicate rotary magnetic heads which altermately reproduce mixed or combined signals from a magnetic tape (not shown) during successive fields and apply the reproduced signals through amplifiers 2A and 2B to a switching circuit 3. Simultaneously, a magnetic head 4 reproduces a control signal from the magnetic tape and applies the reproduced control signal to a servo circuit (not shown) which preforms a tracking servo operation for the magnetic heads 1A and 1B. The control signal from head 4 is also applied to a waveform shaping circuit 5 to produce pulses which are reversed for successive fields and which are supplied to the switching circuit 3 as a control signal for the latter. Thus, the switching circuit 3 is changed over at the commencement of every field and hence delivers therethrough the reproduced signals from the heads 1A and 1B alternately in successive fields. That is, the switching circuit 3 delivers therethrough the reproduced signals continuously.
The reproduced signals from the switching circuit 3 is supplied to a high pass filter 6 which passes therethrough the FM luminance signal and applies the same through a limiter 7 to a demodulator circuit 8 for obtaining the original luminance signal. The original luminance signal is then applied to a mixing circuit 9.
The reproduced signal from the switching circuit 3 is also applied to a low pass filter 11 which passes therethrough the frequency-converted chrominance signal. The chrominance signal from the low pass filter 11 is then applied through an amplifier 12 to a frequency converter 13 by which its carrier is frequency-converted back to the carrier frequency of the chrominance signal of the original frequency band. The converted chrominance signal from convertor 13 is applied through a color killer circuit 14 to the mixing circuit 9 which combines the converted chrominance signal with the luminance signal to produce the original color video signal at an output terminal 10 connected to the mixing circuit 9.
Thus, the color video signal is reproduced by the prior art apparatus. However, since there is generally a scattering in the reproducing sensitivity of the heads 1A and 1B and there is also a lack of uniformity in the contact pressure of the heads 1A and 1B with the tape, the level of the reproduced combined or mixed signal reproduced by the head 1A (refer to FIG. 2A) may be higher than that of the mixed signal reproduced by the head 1B (refer to FIG. 2B). For this reason, the mixed signals from the switching circuit 3 may be changed in level for the successive fields as shown in FIG. 2C. As a result, the level of the chrominance signal from the low pass filter 11 may change for successive fields and, accordingly, the level of the chrominance signal contained in the color video signal delivered to the output termonal 10 may also change for successive fields to produce color flicker in a reproduced color video signal.
In order to avoid such a defect, there is provided in the prior art VTR an ACC circuit 20 which consists of a burst gate circuit 21, a detector circuit 22 and an integrator circuit 23. More specifically, the chrominance signal from the frequency converter 13 is fed to the burst gate circuit 21 which separates or extracts a burst signal. The burst signal is then applied to the detector circuit 22 to be peak-detected and the detected signal is applied to the integrator circuit 23 to provide a DC voltage which corresponds to the peak value of the burst signal and is maintained for one field interval. The DC voltage is applied to the amplifier 12 as its gain control signal. The gain of the amplifier 12 is so controlled by the voltage from circuit 23 that the burst signal contained in the output of the amplifier becomes constant in level. Accordingly, when the burst signal in the chrominance signal from the amplifier 12 is controlled to be constant in level, the chrominance signal is also constant in level. As a result, it would be expected that the level of the chrominance signal in the color video signal delivered to the output terminal 10 is kept constant and hence no flicker would appear in the reproduced picture.
In the prior art VTR, however, since the ACC circuit 20 must correct the level difference between the chrominance signals for successive fields, its response speed is required to be high. However, the burst signal is at most produced at the field frequency and there is a limit on the S/N ratio, so that if the integrator circuit 23 is provided with a short time constant to give the ACC circuit 20 a rapid response, a transient level variation is produced at the initial portion of the chominance signal of each field as shown in FIG. 2D, and such transient level variation appears as color flicker in the reproduced picture. This is uncomfortable for a viewer.